Seen a lot of people spend a lot time trying to blend surfaces on guitar instead of just using “FilletEdge” on the whole closed polysurface. Keep edges clean, Before extrude, definitely keep curve edit points down and delete redundant points before extruding. You should be able to cap the extrude successfully, if it doesn’t cap, clean it up.

Also same principles for fillet edge. Make sure it is closed polysurface. If not closed use “ShowEdges” and show “Naked Edges” and join anything together. If doesn’t work then explode and delete extrusion and “MergeAllEdges” and try again. If still doesn’t work, may need to lower the radius in the fillet edge settings. If still an issue try modifying the cutting objects.

Can be frustrating but it is still way quicker then trying “BlendSrf” each edge separately, find that extremely tedious and can easily be cleaned up with sandpaper. Just make sure nothing is too janky. CAM program will try to cut everything and can lead to damaging your machine. I’ve worked tech support and have seen so many people struggle to understand this. Seen countless time where toolpaths were corrupt due to laziness. Measure Measure cut also applies to CNC.

Paraboloid tool in Rhino is great for belly cuts, can use “Crash” command to make sure it isn’t conflicting with body pockets. Again I had to redo the paraboloid so the fillet would work.